What does it mean?
P1391 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 1 Misfire Detected Catalyst Damaging, Fuel Level Low .
Symptoms
- MIL flashing during the misfire, which warns of catalytic converter damage
- Strong shake at idle and heavy vibration under load
- Loss of power, hesitation and jerking during acceleration
- Fuel gauge reading close to empty when the fault appears
- Fuel smell from the exhaust, sometimes a sulphur odour
- Engine may stall on hard cornering or on a steep grade
Likely causes
- Fuel level so low that the pump pick-up draws air on corners and gradients
- Worn or fouled spark plug on cylinder 1
- Failing ignition coil or a cracked boot on that cylinder
- Clogged, leaking or electrically failed injector on cylinder 1
- Vacuum leak at that intake port or a failing intake gasket
- Low compression on cylinder 1: burnt valve, worn rings or tight valve clearance
- Weak fuel pump or a restricted filter that shows up first at low tank level
How to fix it
- Fill the tank first and see whether the misfire disappears once the pump is properly submerged
- Read the per cylinder misfire counters in live data to confirm cylinder 1 is the offender
- Swap the coil and plug from cylinder 1 with a neighbour and check whether the counter follows
- Inspect the plug for fouling, wear, oil or fuel washing
- Test injector operation with a stethoscope, measure its resistance and compare fuel trims
- Check fuel pressure under load and inspect the filter and tank strainer
- Run a compression and leak down test on cylinder 1 if ignition and fuel check out
- Avoid driving with a flashing lamp, the catalytic converter can be destroyed quickly